Fictive kinship is exactly what it sounds like: fictional kinship. Not the kind you find in novels (well, you may find it in a novel), but rather the kind that has no genetic component: it is, after a fashion, “made up.”
But it would be wrong to consider these relationships anything less than real. Fictive kinship can be profoundly important on an individual or a societal level. Anybody who’s ever seen the term “family of choice” floating around fandom knows that blood isn’t the only thing that can make a relationship narratively powerful; many of you have probably experienced it in real life, too. But fictive kinship isn’t simply a matter of being emotionally close to someone. It’s a formalized connection that’s recognized by the rest of your society.
There’s a sense in which you could call marriage the most common and accepted form of fictive kinship. After all, it creates a new relationship out of thin air, usually (at least these days) with no meaningful degree of blood relationship at all; doesn’t that count? As anthropologists use the term, though, fictive kinship lacks both consanguinity and affinity (i.e. marital connection). It’s basically the “miscellaneous” drawer for relationships that are thought of in familial terms, but aren’t those other kinds of family.
With marriage laid aside, the most widespread and familiar form of fictive kinship is adoption or fosterage. It creates a legal (or at least formalized) parent/child relationship where none existed before. The reasons for this can be manifold: nowadays the most common ones are either that the parents died, or that they’re are unable or unfit to care for their children. But grown adults can be adopted, too. Japanese history offers many examples of families without sons — or with unsuitable sons — adopting someone else to be their heir; sometimes this was a cousin from some more distant branch of the family, but in other cases the adoptee was simply an agreeable young man. If the family had an available daughter, the he might marry her as part of the adoption process, becoming a son-in-law and then a son. The lack of a blood relationship didn’t matter nearly as much in that society as it tended to in Europe, where laws might mandate that only the issue of your body could be counted as your heir.
Fosterage is a slightly different affair. It still creates a type of parent/child relationship, but without transferring legal connection away from the birth parents. The historical model is different from modern foster care; instead of being an alternative to structures like orphanages or an on-ramp to (ideally) formal adoption, it was a way of building social bonds between families. Fosterage might go in a number of directions: in the Celtic regions of the British Isles and in Iceland, higher-ranking individuals fostered their children to the households of their underlings. In other cases, a favored underling might be given the chance to send one of their children for fostering in the household of their superior, giving that child a leg up for future social advancement. Or it might happen between equals, particularly with an eye toward future marriage alliances.
By contrast, marriage alliances have often been prohibited between people related by milk instead of blood. Prior to the invention of modern baby formula, a number of women were employed as wet nurses, i.e. responsible for breast-feeding other women’s children. This could happen because ill health (physical or mental) or insufficient production meant the birth mother couldn’t adequately feed her own baby, but it also happened a fair bit among elite women. Some of them didn’t want to spend their time nursing; others were pressured away from doing so because nursing reduces fertility, and therefore means it will probably be longer before the woman gets pregnant again. (For elite men determined to get a male heir, that was a major concern.)
As with fosterage — of which this can be a type — milk kinship builds social bonds. Choosing a suitable wet nurse was quite an undertaking, because she was responsible not just for the health but often the rearing and education of the child; some people believed her moral character would be passed along in her milk. The lower-status families who fostered a child during the nursing years acquired a special connection to the higher-status family, because they became the fictive kin of that child. This fiction sometimes carried every bit as much weight as a blood relationship: Islam prohibits sexual contact between a man and his wet nurse, and some branches extend that to all the immediate consanguineous kin of the wet nurse, so that (for example) a man cannot wed his wet-nurse’s daughter, either. She is effectively his sister. “Milk siblings” are especially a thing when two children were nursing at the same time, one fostered, one the wet nurse’s own child.
Then you have godparents, whose role varies widely depending on which community you’re looking at. It’s largely a Christian concept, though there are somewhat analogous things in other religions; they stand as formal witnesses during baptism, and afterward are considered to have a lasting responsibility toward the one baptized. The familial nature of that relationship particularly comes to the forefront when you realize that godparents are prohibited from marrying their godchildren, just as if they were related by blood.
Fictive kinship can go beyond these familiar categories. My parents never fostered my best friend, much less formally adopted her, but somewhere along the line they took to introducing her as their “other daughter,” and I refer to her as my sister often enough that it was weird for me to type “my best friend” at the beginning of this sentence. Other kinds of mentorship beyond the spiritual responsibility of a godparent can acquire significance; the “cap parent” or eboshi-oya in the Japanese coming-of-age ceremony could be very important, and the child might use one of the characters from the cap parent’s name in their own adult name. Speculative fiction has even more room to invent new types of relationship that could fit into this category — what if your magic was somehow inherited from another, unrelated person, rather than through family lines? The possibilities are endless.
Do you have any fictive kin of your own? Or know anyone who does?